Concentrator system for particulates suspended in air

ABSTRACT

Air carrying particulate matter in suspension enters horizontal plenum at high velocity; bag filters along top of plenum are connected through openings the aggregate cross sectional area of which is much greater than the cross sectional area of the plenum, so that velocity of air rising into bags is relatively low. Plenum has gradual reduction of cross section so that horizontal air velocity through it is substantially uniform along its length, keeping particles entrained, sweeping them along the smooth bottom and through the outlet. Momentary interruption of inlet air reduces pressure in bags, allowing partial collapse which causes trapped particles to fall into plenum, from which they are discharged when air flow is restored. Connection of the discharge outlet of one such unit into the inlet of a second unit, in addition to the normal input load of the second unit, reduces the proportion of the ultimate particlebearing exhaust in terms of the input to the two units.

United States Patent Brandt Feb. 4, 1975 CONCENTRATOR SYSTEM FOR PARTICULATES SUSPENDED IN AIR 1,234,026 6/1971 Great Britain 55/341 Primary ExaminerBernard Nozick [75] Inventor: Robert E. Brandt, Delavan, W1s. Attorney, Agent or Firm David R Bait; F M [73] Assignee: Bunker Ramo Corporation, Oak Arbuckle Brook, Ill. 22 Filed: July 18, 1973 i5 7i ABSTRACT.

Air carrying particulate matter in suspension enters PP 9103380520 horizontal plenum at high velocity; bag filters along top of plenum are connected through openings the ag- 52] US. Cl ss/zss, 55/97, 55/324, gregaie Cross Sectional area of which is much greater 55/334 55/341 55/350 55/4 1 55/473 than the cross sectional area of the plenum, S0 that ve- 51 Int. Cl BOld 50/00 iociiy Of air rising into bags is relatively Plenum 5 Fie|d f Search 55 97 1 286488 has gradual reduction of cross section so that horizon- 55 321 3247 3347 335437, 3417 4617 tal air velocity through it is substantially uniform along 55/4l8, 430, D10 37, 350, 473 its length, keeping particles entrained, sweeping them along the smooth bottom and through the outlet. Mo- [56] References Cited mentary interruption of inlet air reduces pressure in UNITED STATES PATENTS bags, allowing partial collapse which causes trapped particles to fall into plenum, from which they are dis- Ruemelm 55/341 charged when air flow is restored 2:273:793 4/1942 Connection of the discharge outlet of one such unit 3,535,851 10/1970 Riemsloh 55/341 into the inlet of a second unit, in addition to the 3,538,688 11/1970 Stanley, Jr. et a1. 55/418 normal input load of the second unit, reduces the 3,546,852 12/1970 Furstenberg et al 55/341 proportion of the ultimate particle-bearing exhaust in FOREIGN PATENTS OR APPLICATIONS terms of the Input to the two units.

1,576,547 6/1970 Germany 55/430 7 Claims, 4 Drawing Figures IT'S; a I

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sum 3 or PATENTED FEB 975 t l illlll lul CONCENTRATOR SYSTEM FOR PARTICULATES SUSPENDED IN AIR BACKGROUND OF THE INVENTION Manufacturing operations which generate stray particulate matter must have some system for removal of the particulate matter from the areas in which working takes place. Both personal health and safety considerations require the removal of dusts and fibers from working areas, and purely mechanical considerations necessitate the removal of material which may accumulate and cause blockage or obstruction of machine parts in a way likely to interfere with their proper functioning. An increasingly more important consideration is the recovery of materials which have reuse or salvage value.

A common method of removing such matter is by exhaust systems which utilize fans to draw air through pipes or ducts having inlets near the points of particle generation. The object is to entrain the generated particles in the air flow, and thus to carry them to some point at which they can be removed from the air stream by mechanical entrainment between the inter stices of a porous mesh filter.

Where'large volumes of air are required to gather and move the amounts of particulates involved, an economic problem arises in circumstances where either heating, cooling or moisture control of the factory air is necessary to provide satisfactory working conditions. If the air used for carrying particulate matter is exhausted to the outside of a factory building, equivalent quantities of air from outside must be heated (or cooled) to take its place, It is therefore highly desirable to provide a system in which most of the air drawn from work areas within the factory is returned to the factory so as to minimize the cost of heating or cooling makeup air.

To remove fine particles from an airstream, it has long been common to use a bag filter. This is equivalent to the common vacuum cleaner bag, a tubular cloth member arranged so that dust laden air can be passed into it. The air passes through the meshes of the cloth structure, leaving most of the dust particles trapped in or on the mesh. The bags are usually suspended with their long axes vertical, so that they can be shaken periodically to drop out the accumulated dirt. The shaking causes wear and ultimately failure of the bags, both at suspension points and at any point where the bag is contacted by shaking mechanism. This has resulted in various attempts to shake the bags by sound waves (U.S. Pat. No. 3,053,031), by internal airjets (U.S. Pat. No. 3,325,979) or by external air currents (US. Pat. No. 3,383,840).

In the textile industry, where the particles are of fibrous nature, additional problems are involved because of the relatively low density of the fibers, and the fact that their shape exposes a considerable amount of surface for their weight, so that they tend to settle relatively slowly. Furthermore, they are made with a twist, for the very purpose of increasing the interengagement between individual fibers in a yarn or thread. As aresult of this twist, the longer fibers catch easily on 'obstructions, discontinuities or projections in air passageways, and as one fiber traps another, a blockage can build up.

Filtering systems are known, for use in cleaning the intake air for internal combustion engines, employing reversal of flow direction to drop out larger particles, and porous paper disposable filters to trap fine particles (US. Pat. No. 3,378,994). Such a system is not appropriate for the recovery of textile fibers; because oftheir low density, the fibers are not thrown out of the airstream very satisfactorily by direction-change techniques, and they tend to hang,up and form matted blockages upon and between closely spaced directionchanging baffles introduced into the airstream.

In textile manufacturing industries, particularly in the making of deep pile fabrics, a small percentage of fiber goes astray at the processing machines and must be recovered by an appropriate collection system. Deep pile circular knitting machines differ from plain circular knitting machines principally in the respect that various stations are established around the knitting machine needle cylinder for feeding fiber taken from a rope or sliver of such fibers. At each station around the machine, the sliver is combed and carded into individual fibers an inch or more long which are introduced into the needle hooks, their direction being changed by air jets as each individual knitting stitch of the backing yarn is made.

As a result, the jersey knit fabric made up of the backing yarn has locked within it bundles of deep pile fibers so as to produce a fabric sometimes known as fake fur. The pile which is attached to the backing is, after the initial knitting, processed by shearing, polishing and other steps, to produce various desirable characteristics for the end product.

In the knitting operation, the needle hooks move up into the carded sliver, and retract with a number of the fibers caught in the hook. As the hook moves down for completion of the stitch, an air blast directed toward the center ofthe machine plays against the hook, carrying all the fiber ends toward the center. The pile is thus made to be all on one side ofthe goods. Until the stitch is completed, however, the fibers are somewhat loosely held in the needle hook, and it is inevitable that some of them come free and are carried off in the air blast. Hence it is necessary to have an exhaust system to collect such fibers and remove them from the knitting machine room.

In subsequent stages of the manufacturing process, the pile is sheared (and sometimes resheared) to give a uniform height of pile, polished (by passing under heated, bladed drums which vigorously stroke the fibers) and subjected to various other finishing treatments, many of which generate fragments of fiber or other by-products which require exhaust collection and removal from the work area.

Fibers from the knitting machines are typically one or more inches in length, and have a higher salvage value than the shorter clippings from shearing and other finishing operations. For this reason separate collection systems are appropriately used, but for each, the general objects are the same to remove the stray material from the working area, to deposit it at an appropriate collection point, and to return the exhaust air, minus stray material, to the working area.

The present invention provides a particle recovery system for closed air conditioned work rooms in which bag filters are used in a way to provide very simple maintenance. As will subsequently be described in more detail, cleaning of the bag filters can be accomplished without a shutdown of the exhaust system which would interrupt factory operations.

SUMMARY OF THE INVENTION The invention resides in a system utilizing a horizontal duct or plenum through which exhaust system air carrying entrained particles is passed at a substantially uniform high velocity. A major part of the air entering the duct leaves it through bag filters along the top of the duct, which are joined to it by connections having an aggregate cross section much greater than the cross section of the duct, so that air rises into the bag filters very slowly, and carries few particles up with it. The cross-section of the duct is progressively reduced along its length, so that as the total volume of air moving through it decreases, the horizontal velocity will be maintained all the way to the outlet, where the remaining air, (a small part of that entering the duct) carries the entrained particulate matter through the outlet into a collection bin. Tandem operation, discharging into a duplicate system, permits a reduction in the proportion of exhaust air to total air used for entrainment.

DESCRIPTION OF THE DRAWINGS The foregoing and other features of the invention will be apparent from the following detailed description of a preferred embodiment thereof, and .by reference to the accompanying drawings, in which FIG. 1 is a diagrammatic representation of a particle concentration system as applied in a factory making knitted deep pile fabrics;

FIG. 2 is an elevational view ofa bag filter unit which is a part of the invention;

FIG. 3 is a plan view of the same; and

FIG. 4 is a perspective view of a fragmentary portion of the filter structure.

DETAILED DESCRIPTION In the diagramatic view of FIG. 1, the reference numeral indicates generally a factory building enclosure, within which the ambient air may be heated or cooled, humidified or dehumidified in order to provide appropriate working conditions, both in terms of worker comfort, and in terms of optimum conditions for the processes being carried out in this case a deep pile textile manufacturing operation.

In the knitting area lying under the bracket 12, one group of knitting machines 14 is served by an exhaust system which includes hoods 16 and branch pipes 18 leading into a main pipe 20, which passes through a wall 22 into the filter room 24. A centrifugal fan 26 draws air and particulate matter entrained therein through the system described, and passes it into a bag type filter designated generally as 28. A similar system serves a group of knitting machines 30, with a main pipe 32 leading to a fan 34 and a filter 36.

Likewise, a group of preshear machines typified by 38, 40 and 42 are exhausted into a filter 44, and finishing machines 46, 48 and 50 are exhausted into a filter 52.

The filters 28, 36, 44 and 52 are in accordance with the present invention made in such a way as to perform the desired filtering operation with a minimum of maintenance. A typical filter arrangement is shown in FIG. 2 in elevational view. The main exhaust pipeline 54 leads into a centrifugal fan 56, the outlet 58 of which is so arranged that the outside of the scroll casing is the bottom of the outlet. Thus, the effect of centrifugal forces moving the entrained particles toward the outside of the fan casing, tends to concentrate them along the bottom of the outlet. The outlet connects through a transition section 60 to the entrance 72 of a wide, shallow, smooth bottom duct or plenum 62. The plenum should be substantially free of obstructions, projections or abrupt changes in contour, particularly along the bottom; such discontinuities would catch fiber particles or cause unnecessary turbulence in the air stream. It is desirable that the flow of entrained particles which have been thrown to the bottom of the duct by the fan should be disturbed as little as possible in any way which would tend to increase the particle concentration in the upper part of the plenum.

In the example shown, the section A of the filter has a plenum of uniform width, (FIG. 3) communicating with 28 filter bags 64. The filter bags are closed at their top ends, and suspended in any suitable manner. The height of the plenum is tapered, being at the outlet end of section A about one-third the height of the entrance to section A.

The plenum should not be made too shallow, which could lead to clogging by balls or bunches of fiber. The reduction in height of the plenum should not be carried below a practical minimum, which will depend upon the nature and quantity of the particulate matter being carried. Therefore a section B of the plenum is made uniform in height, but the reduction in cross-section area is continued by tapering the width, and the number of bags which can be accommodated across a section of the plenum is correspondingly reduced toward the outlet end, as may be seen in FIG. 3.

The purpose of the progressive reduction in cross section of the plenum from inlet 72 to outlet 88 is to equalize the amount of air which flows into each of the several filter bags along the way, and to maintain as nearly as possible a uniform velocity of the air and entrained material moving with it along the plenum.

It is a further purpose to carry the particulate matter horizontally out of the plenum continuously, using as small a part of the original entering air as practical. This amount ofair will vary somewhat with the amount and nature of particulate matter being collected, but with textile fibers it can be done with about 10 to 15 percent of the entering air. For this reason, the area of the outlet 88 of section B is about one tenth the area of the entrance 72 to section A, entrance and exit velocity being the same.

It is an object of this filter system to minimize the amount of particulate matter which passes into the filter bags, in order to reduce, in turn, the frequency of cleaning the bags, and consequently the rate at which they wear out and require replacement.

With this object in view, the air velocity in a horizontal direction along the plenum is maintained at a fairly high level, typically 4,000 to 4,500 feet per minute. Air moving at this velocity keeps the fibers entrained, and sweeps them along the smooth bottom of the plenum from one end of the filter to the other. On the other hand, air moving up into the filter bags comes from the upper strata ofthe air moving through the plenum, having a lower concentration of particulate matter. The upward movement into the bags is at a very low velocity, as will be explained, and consequently the air moving upward carries very little fiber with it.

Details of a construction suitable for the top of the plenum are shown in FIG. 4. Almost the entire top of the plenum is actually open into the filter bags. Horizontal structural members 70 extend transversely at the top of the plenum, and support a plurality of transitional members 74, each of which provides transition from a rectangular opening (preferably square) at its lower end, to a circular collar 76 at its top, around which a filter bag 64 is secured by a clamping band 80. The top end of the bag 64 is closed and suspended in any suitable manner.

In section B of the plenum, the tapering in width requires that small portions of the top be closed, as at 78a, 78b and 78c, but the greater portion is open into the filter bags, as in section A.

To maintain the velocity of air leaving the plenum, and to provide a means of controlling the static pressure on the last few bags in the filter, a small centrifugal fan 84 isprovided, being connected by a transition piece 86 to the outlet 88 of the plenum.

With the arrangement described, it will be observed that the open area of the top of the plenum is much greater than the cross-section of the plenum at its entrance, and this is an essential feature of the invention. It assures that even with a rather high velocity of movement of air horizontally into and through the plenum, the vertical velocity of air movement out of the plenum into the filter bags will be relatively low.

Typical figures for an installation will make this clear.

The entrance to section A of the plenum is a rectangle 9 inches high, 62 inches wide, having an area of 3.875 square feet, and will pass 15,500 cubic feet per minute of air moving at the recommended minimum velocity of 4,000 feet per minute.

In a filter unit of such width with seven rows of bags, section A is about 9 feet long, and the inside height at the end of the section is 3 /2 inches, with an area of 1.5 square feet. Section B, using 12 transition sections 74 of the same size, arranged as shown in FIG. 3, has a length of about 8 feet, and an outlet 3 /2 inches high, 16 inches wide, with an area of 0.39 square foot.

The arrangement described has 40 filter bags, entered through transition sections approximately 15 /2 inches square at the base, and each having therefore an entrance cross-section of about l,6 square feet. The total cross-section of the 40 outlets is thus 64 square feet, more than 16 times the area of the entrance to section A of the plenum. If 90 percent of the entering air passes out through the filter bags, through 16 times the entrance area, the average velocity of air moving vertically into the transition sections 74 will be 90 percent of one-sixteenth of the 4,000 feet per minute velocity at the entrance to section A, or about 225 feet per minute. This is a speed of about 2 /2-miles per hour; literally, a breeze of that speed will not raise much dust. This velocity is so low that only a very small amount of fiber will be carried into the filter bags out of the 4,000 feet per minute flow along the plenum.

Both gravity and inertia aid the separation process at this point. The textile particles are more dense than the air in which they are suspended. The odds are against their changing direction from their horizontal movement in the plenum airstream; of any possible directional change, they are least likely to go in an upward direction, and the air with low vertical velocity is not an efficient carrier. The result is that very little fiber is carried by the air passing into the bags.

Such fiber as is carried up into the bags is held lightly against the walls ofthe bags by the gentle current of air passing through them. Much of it will fall by gravity down into the plenum when the amount ofincoming air is momentarily reduced. To some extent, this might be said to take place automatically; in a textile fiber exhaust system there can be a considerable amount of fluctuation in the air flow, due presumably to the fact that slugs of matted fiber are occasionally dislodged from points of accumulation, and travel through the pipeline less freely than the normally floating fibers, causing pulsations of pressure in the line. But for purposes of routine cleaning of the filter bags, the effect can be produced by complete or partial closure of a damper or blast gate in the pipeline 54. This causes the flow of air through the walls of the bags to be reduced, largely removing the force which holds the accumulated fibers against the walls. In addition, the bags change shape as the static pressure within them drops; they may even wrinkle somewhat and have an inflow of air if the outlet fan 84 brings their internal pressure below room pressure. All of these effects tend to cause fibers to fall away from the inner wall surfaces of the bags and to drop into the plenum, from which they are swept out through the outlet 88 when normal air flow is restored.

Closure of the gate 90 needs to be only very brief, and may be repeated several times to give a pulsating pressure within the filter bags. But the reduction of exhaust function is for such a short time that no interruption of production operations results, and there is no necessity, as in other systems, for a standby filter which can be put into service while the first one is cleaned.

There are some advantages, however, in dividing the total filter capacity for a plant into subsystems. In FIG. 1 it will be noted that two subsystems, utilizing filter units 28 and 36, are provided to serve the knitting machine operations, which typically generate a high proportion of fibers of an inch or more in length. Two other subsystems, utilizing filter units 44 and 52, serve the shearing and other finishing operations, which typically generate a high proportion of fibers less than an inch in length. The salvage value of the fibers from knitting machine operations is considerably higher, so that it is advantageous to be able to segregate this material, as will be further described.

Where two subsystems are used for collection of the same kind of particulate matter, another advantage arises. For example, the air from the outlet of filter unit 28 can be directed by the pipe 92 to the intake for filter unit 36. The air passing through the outlet 88 of filter unit 36 will then contain substantially all the particulate matter which entered filter units 28 and 36, but only about 5 percent of the total air which entered those two filter units. This air is then exhausted by the fan 84 through an outlet 94 directly into the collection room or bin 96, which is separated from the filter room 24 by wall 98. This wall is not the full height of the filter room 24, but of some lesser height which gives adequate working space in the collection room 96. The collection room has a ceiling consisting of a suitable filter cloth 100, shown in part in FIG. 1.

Air discharging from the fan 84 through the outlet 94 passes directly into the room 96. Because of the abrupt passage into the room area, its velocity drops sharply, and the particulate matter settles out, most of it falling into a hopper 102 in the floor, and moving from there to baling machines or other disposal apparatus.

Filter unit 28 can be provided with an alternative discharge opening 104, fromwhich its output can be discharged directly into the collection room 96 and hopper 103, either in the case where the material being collected is to be segregated from that from other subsystems, or in case filter unit 36 should be out of service for any reason.

It will be noted that the piping of filter units 44 and 52 includes a pipe connection 106 which enables these two units also to be used in the manner just described, discharging thesolid matter from both into hopper 108, or the output from each separately, into hoppers 110 and 108.

As will be understood, the advantage of using two filters in succession is the much greater degree of concentration of the particulate material which is obtained, and the relatively small amount of air which needs to be passed into the collection room 96. With a minimum amount of incoming air, the velocities of air movement within the room are generally low, so that the particles will settle'out readily, and only a minimum will be carried up to the cloth ceiling filter 100.

All air entering the collection room 96 must pass out through the ceiling filter 100, and thence over the wall 98 into the filter room 24. I

All air entering the filter room 24, either from collection room 96 or through theair bags 64 of the various filter units, is returned to the factory working area through suitable final filters 118, and by way of fans 120 and distribution ducts 122.

There is thus provided a concentrator system for particulates suspended in air which permits segregation of various kinds of particulate matter, requires a minimum of labor for operation and for cleaning, permits cleaning without interruption of the production operations which it serves, and does not require otherwise unused standby filters to provide such uninterrupted service. In addition to these advantages, all air used is completely recycled, thus eliminating the cost of conditioning makeup air.

Theinvention claimed is:

1. A concentrator system for particulate matter suspended in air including intake piping for collecting particulate matter,

a fan for moving air through said piping,

a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, and

a plurality of filter bags communicating with and connected to said apertures,

the cross section area of said plenum being uniformly reduced from the inlet to the outlet thereof, in two sections, the first section being of uniform width, and of uniform gradation in height, and the second section being of uniform height and uniform gradation in width.

2. A concentrator system in accordance with claim 1, further including means whereby the air moving effect of said fan can be momentarily and repeatedly reduced to cause a pulsation of static pressure within said plenum.

3. A concentrator system for particulate matter suspended in air including intake piping for collecting particulate matter,

a fan for moving air through said piping,

a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, said apertures each opening into a filter bag, and said fan being a centrifugal type adjoining said inlet and being so arranged that the outer periphery of the scroll casing thereof is at the bottom of its outlet and is connected to the bottom of the inlet of said duct whereby particulate matter centrifugally thrown to the outer portion of said scroll casing by the operation of saidfan is introduced at the bottom of said duct, and moves from the inlet to the outlet of said duct along the bottom thereof.

4. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced,

an outlet at the other end leading to a collection chamber, the area of said outlet being approximately one tenth of the area of said inlet,

a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and

a plurality of apertures in the top wall thereof, having an aggregate area at least 10 times the area of said inlet, and

a plurality of filter bags connecting with said apertures.

5. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced,

an outlet at the other end leading to a collection chamber,

a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and

a plurality of apertures in the top wall thereof, and

a plurality of filter bags connecting with said apertures,

the vertical velocity of air rising through said apertures being less than 400 feet per minute when a horizontal air velocity of 4,000 feet per minute is maintained in said plenum.

6. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced,

an outlet at the other end leading to a collection chamber,

a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and

a plurality of apertures in the top wall thereof, and

plurality of filter bags connecting with said apertures, said duct having a first section of uniform width and of uniform gradation in height, and a second section of uniform height and uniform gradation in width.

7. A concentrator system for particulate matter sus pended in air including intake piping for collecting particulate matter,

a fan for moving air through said piping,

a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, and a plurality of filter bags communicating and connecting with said apertures, the cross section area of said plenum being uniformly reduced from the inlet to the outlet thereof, said piping being directed vertically, with a downward flow of air, adjacent to said filter unit, and entering said filter unit through means receiving the downward flow of air and discharging it horizontally into said plenum. 

1. A concentrator system for particulate matter suspended in air including intake piping for collecting particulate matter, a fan for moving air through said piping, a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, and a plurality of filter bags communicating with and connected to said apertures, the cross section area of said plenum being uniformly reduced from the inlet to the outlet thereof, in two sections, the first section being of uniform width, and of uniform gradation in height, and the second section being of uniform height and uniform gradation in width.
 2. A concentrator system in accordance with claim 1, further including means whereby the air moving effect of said fan can be momentarily and repeatedly reduced to cause a pulsation of static pressure within said plenum.
 3. A concentrator system for particulate matter suspended in air including intake piping for collecting particulate matter, a fan for moving air through said piping, a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, said apertures each opening into a filter bag, and said fan being a centrifugal type adjoining said inlet and being so arranged that the outer periphery of the scroll casing thereof is at the bottom of its outlet and is connected to the bottom of the inlet of said duct whereby particulate matter centrifugally thrown to the outer portion of said scroll casing by the operation of said fan is introduced at the bottom of said duct, and moves from the inlet to the outlet of said duct along the bottom thereof.
 4. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced, an outlet at the other end leading to a collection chamber, the area of said outlet being approximately one tenth of the area of said inlet, a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and a plurality of apertures in the top wall thereof, having an aggregate area at least 10 times the area of said inlet, and a plurality of filter bags connecting with said apertures.
 5. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the Air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced, an outlet at the other end leading to a collection chamber, a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and a plurality of apertures in the top wall thereof, and a plurality of filter bags connecting with said apertures, the vertical velocity of air rising through said apertures being less than 400 feet per minute when a horizontal air velocity of 4,000 feet per minute is maintained in said plenum.
 6. In an air filter system wherein a flow of air in a duct carries particulate matter which is to be removed from the air, a filter unit comprising a plenum formed by a duct extending substantially horizontally between opposite ends, having an inlet at one end into which air bearing particulate matter is introduced, an outlet at the other end leading to a collection chamber, a bottom therebetween sufficiently smooth for particulate matter to be swept therealong to and through said outlet by said air, and a plurality of apertures in the top wall thereof, and a plurality of filter bags connecting with said apertures, said duct having a first section of uniform width and of uniform gradation in height, and a second section of uniform height and uniform gradation in width.
 7. A concentrator system for particulate matter suspended in air including intake piping for collecting particulate matter, a fan for moving air through said piping, a filter unit comprising a plenum receiving from said fan air and particulate matter entrained therein, said plenum being formed by a duct having an inlet at one end for horizontally moving air and an outlet at the other end, substantially smaller than said inlet, for horizontally moving air, and a plurality of apertures along the top wall of said duct between said inlet and said outlet, for vertically moving air, and a plurality of filter bags communicating and connecting with said apertures, the cross section area of said plenum being uniformly reduced from the inlet to the outlet thereof, said piping being directed vertically, with a downward flow of air, adjacent to said filter unit, and entering said filter unit through means receiving the downward flow of air and discharging it horizontally into said plenum. 